Demand for cellular telephone service continues to grow exponentially, with market analysts presently projecting that over ten million cellular telephones will be in operation in North America by 1992. As a result of this demand, it can be expected that the market for cellular subscriber equipment will continue to be quite competitive.
Increasingly, consumers are willing to pay more for a portable, hand-held cellular telephone ("hand-held"). This is primarily because a hand-held telephone is more versatile than a mobile cellular telephone ("mobile") intended for permanent installation in a vehicle.
In order to remain competitive, however, a hand-held must be as physically small and lightweight as possible. Because hand-helds are purchased at a premium customers also expect excellent quality transmission and reception. Accordingly, the electronic radio frequency (RF) components used in a handheld must be extremely efficient, not only in the range of functions provided in a given physical volume, but also in terms of power dissipation, since hand-helds invariably operate on battery power.
While adequate efficiency is easily achieved at low carrier frequencies, such as those in the high-frequency (HF) band traditionally reserved for conventional mobile telephone service, such is not the case for a cellular telephone, which typically uses carrier frequencies in the Very High Frequency (UHF) band, above 800 megahertz (MHz). Current designs provide adequate operation at these frequencies if the RF circuits are fairly narrowband. Since a cellular transceiver must be capable of operating on any one of hundreds of channels upon command from a base station, its required operating bandwidth usually exceeds 25 MHz. This is not considered to be a particularly narrow bandwidth.
Typically the higher the frequency, and the wider the bandwidth of an RF component, the less efficient it is. Reduced RF component efficiency translates directly into an increased demand on the power supply. This efficiency dilemma is particularly troublesome to the designer of a hand-held transceiver, since power consumption must be minimized if the battery is to be as small and lightweight as possible.
Another RF design consideration in a cellular telephone is the duplexer. The duplexer allows the transmitter and receiver to operate simultaneously, and hence allows the user to talk and listen at the same time, as with a conventional telephone. This so-called duplex operation typically requires that the transmitter operate at a different radio frequency than the receiver.
Thus, where there is only one antenna, duplex operation requires the transmitter and receiver to share the antenna. This sharing is accomplished by a duplexer, which is a three-port filter coupled to the antenna, the receiver, and the transmitter. The duplexer prevents transmitter RF signals from damaging or interfering with the receiver. Thus, it provides a low impedance path from the transmitter to the antenna for signals at the transmit frequency, and a high impedance path from the transmitter to the receiver, so that the receiver is isolated from the transmit signals. The duplexer also provides a low impedance path between the antenna and receiver for signals at the receive frequency, and a high impedance path between the receiver and transmitter, so that the transmitter is isolated from the receive signals.
The duplexer presents a problem to the designer of a cellular transceiver because of the required proximity of the transmitter and receiver bands, broad bandwidth, and high isolation. In fact, these requirements cannot usually be met without a multiple pole bandpass filter positioned in the transmit signal path. The need for a filter with multiple poles between the transmitter and the antenna in turn means that a fairly large insertion loss must be accepted. This results in reduced transmitter efficiency, and a corresponding increase in the amount of power which the battery must provide.
Because of these and other design requirements, a duplexer is often the most expensive single component of a hand-held cellular telephone.
Consider another RF component, the antenna itself. Generally speaking, as the operating frequency of an antenna is increased, its sensitivity to perturbation by the surrounding environment is also increased. At present, most hand-held cellular telephones use monopole, or so-called "whip", antennas. However, the gain of a whip antenna is noticeably reduced by the proximity of a human body. This is indeed another perplexing problem to the designer of a hand-held cellular telephone, since the hand-held necessarily must be used in such a fashion as to bring the antenna extremely close to the user's head. The transmitter in such a unit must normally be designed to have sufficient reserve power to overcome the loss presented by the user's head.
Another consideration in the design of an antenna for hand-helds is that RF radiation into the head of the user should be minimized. Such is not always the case with various whip antenna designs.
Whip antennas are also considered to be a nuisance, whether they are of the fixed-geometry or retractable type. Fixed-geometry whip antennas tend to break, and are often in the way when the hand-held must be stored. Retractable whip antennas must be extended to operate the hand-held and then retracted after use.
Some have proposed compact antenna structures for high frequency portable radio operation. See, for example, Taga, et al., "Performance Analysis of a Built-In Planar Inverted F Antenna for 800 Mhz Band Portable Radio Units", IEEE Journal on Selected Areas in Communication, Vol. SAC-5, No. 5, Jun. 1987, pp. 921-929, and Kuboyama, et al., "UHF Bent Slot Antena System for Portable Equipment-I", IEEE Transactions on Vehicular for Technology, Vol. VT-36, No. 2, May 1987, pp. 78-85.